Counting of Sulphate-reducing Bacteria in Mixed Bacterial Populations

Abstract

IN the dilution method of counting Desulphovibrio in soil or water, a selective medium containing lactate and inorganic salts is commonly employed. The growth of a sulphate reducer is recognized by the appearance of the black colour of ferrous sulphide. Since the organisms are anaerobes and the medium is simple in composition the addition of a reducing agent is essential. Grossman and Postgate¹ selected cysteine as the most suitable of the reducing substances they examined. Cysteine possesses the disadvantage that it may be attacked with the liberation of sulphide by certain bacteria which do not reduce sulphate. Hence it is necessary to apply some test to confirm the presence of Desulphovibrio in cultures which become black. A further disadvantage in the method proposed by Grossman and Postgate is that the cultures are incubated in an anaerobic atmosphere.